The present invention relates to a fiber optic cable used in a wavelength division multiplexing (WEM) optical transmission system, and more particularly to a fiber optic viable capable of suppressing influences, caused by a non-linearity of optical fibers, to a maximum while controlling dispersion characteristics to an appropriate level in order to obtain a maximum transmission capacity per optical fiber. Also, the present invention relates to WDM optical transmission system using the fiber optic cable, and more particularly to a WrM optical transmission system which can operate efficiently even when it uses a reduced channel spacing for an increase in transmission capacity.
Optical transmission techniques using optical fibers have rapidly been developed in that they can transmit a large quantity of data within a short period of time while involving a reduced transmission loss. In particular, such optical transmission techniques have further been advanced by virtue of development of a new optical fiber capable of transmitting signals for a long distance.while involving a reduced signal loss, and development of a superior light source such as a semiconductor laser.
However, known optical fibers involve a chromatic dispersion, that is, a phenomenon in which a signal is spread due to a difference in group velocity among the wavelength components of the signal. Due to such a chromatic dispersion, a signal overlap occurs at the receiving terminal, thereby resulting in a fatal problem such as an impossibility of demodulation. For this reason, attempts to minimize such a chromatic dispersion (hereinafter, simple referred to as a xe2x80x9cdispersionxe2x80x9d) have been made. By virtue of such attempts, it has been found that a zero dispersion is achieved at an operating wavelength of 1,310 nm.
Meanwhile, it has been found, on the basis of the relation between the total loss and the wavelength in an optical fiber, that a minimum signal loss is exhibited at a wavelength of 1,550 nm even though an increased dispersion occurs, as compared to that occurring at 1,310 nm. In this connection, the operating wavelength of 1,550 nm could be used by virtue of the development of a new optical amplifier capable of amplifying the wavelength range of 1,530 nm to 1,565 nm. As a result, a non-repeating long distance transmission has been possible. This has resulted in the advent of a dispersion-shifted fiber (DSF) adapted to shift the zero dispersion from the wavelength of 1,310 nm, at which the zero dispersion is achieved in conventional cases, to the wavelength of 1,550 nm in order to obtain a minimum dispersion and a minimum signal loss.
In addition to such a development of optical fibers, a WDM system has been developed, which serves to multiplex a plurality of optical signals having different wavelengths so as to simultaneously transmit those optical signals through a single optical fiber. Using such a WDM system, it is possible to more rapidly transmit an increased amount of data. An optical communication system using a WDM scheme at a wavelength of 1,550 nm has already been commercially available.
Where the above mentioned DSF is used in such a WDM optical transmission. system, however, a signal distortion may occur even though a desired zero dispersion may be achieved. This is because the zero dispersion in the optical fiber may result in a non-linearity of the optical fiber, for example, a four-wave mixing in which lights of different wavelengths may be mixed together, even though a reduced signal distortion is obtained.
In particular, the most practical method usable in the WDM optical transmission system for a further increase in transmission capacity is to increase the number of channels used. In order to increase the number of channels used, however, it is necessary to use a reduced channel spacing because optical amplifiers use a limited amplification band. Such a reduced channel spacing may result in a more severe problem associated with the non-linearity of the optical fiber such as the four-wave mixing.
The non-linearity of an optical fiber is reduced at an increased channel spacing or an increased dispersion of the optical fiber. However, where the dispersion of the optical fiber increases, a degradation in transmission quality occurs inevitably due to a distortion of optical signals resulting from the increased dispersion.
Therefore, it is necessary to control the dispersion of the optical fiber in order to obtain a maximum transmission capacity of the WDM optical transmission system. In other words, an excessively high dispersion results in an increased signal distortion whereas an excessively low dispersion approximating to the zero dispersion results in a non-linearity of optical signals such as the four-wave mixing phenomenon, thereby generating a signal degradation. In this regard, it has been strongly required to develop an optical fiber capable of solving both the problem resulting from the dispersion and the problem resulting from the non-linearity.
U.S. Pat. No. 5,327,516 issued on Jul. 5, 1994 discloses an optical fiber for a WDM system which exhibits a dispersion ranging from 1.5 ps/nm-km to 4 ps/nm-km at a wavelength of 1,550 nm in order to achieve a suppression in non-linearity. The optical fiber disclosed in this patent is called a xe2x80x9cnon-zero dispersion-shifted fiber (hereinafter, referred to as an xe2x80x9cNZ-DSFxe2x80x9d) in that it is configured to obtain a non-zero dispersion. Such an optical fiber is commercially available from Lucent Technologies In., U.S.A.
The NZ-DSF is significant in that it can suppress the four-wave mixing phenomenon by virtue of its dispersion value ranging from 1.5 ps/nm-km to 4 ps/nm-km. However, where a long-distance transmission is carried out using such an NZ-DSF or an increased number of channels, it is difficult to compensate for a surplus dispersion increased by a dispersion slope on the increased number of channels even though the dispersion accumulated on one channel may be compensated for by use of a dispersion compensation module (DCM) with a high negative dispersion value.
Furthermore, the NZ-DSF exhibits a relatively low dispersion value while having a relatively small effective area of 55 xcexcm2(in the case of a single-mode optical fiber, it has an effective area of about 80 xcexcm2. Since the effective area of the optical fiber is the actual area of an optical signal within the optical fiber, the optical signal has a reduced density for the same optical power as the optical fiber has an increased effective area. At a reduced density of the optical signal, the optical fiber exhibits a relatively reduced non-linearity. In this regard, where a very narrow channel spacing is used, it is difficult to sufficiently suppress the four-wave mixing phenomenon in the NZ-DSF with a relatively small effective area.
In particular, current WDM optical transfer systems show a tendency to use a channel spacing gradually reduced from 200 GHz to 100 GHz, and to 50 GHz. Such a tendency is due to the necessity of an increase in transmission capacity. However, where a very narrow channel spacing of 50 GHz is used, it is difficult for the NZ-DSF to be applied to WDM long-distance optical transmission systems.
FIG. 2a schematically illustrates an example of a WDM optical fiber system using an NZ-DSF. The illustrated optical fiber system, which is denoted by the reference numeral 20, has a channel spacing of 50 GHz and 8 channels. This optical fiber system 20 receives optical power of 0 dBm per channel from a light source. NZ-DSFs 24 are distributed over a total distance of 480 km. A dispersion-shifted optical fiber (DCF) 25 is also arranged in every span, along with an optical amplifier 23. The detailed specification of the optical transfer system 20 illustrated in FIG. 2a is described in the following Table 1.
The optical transmission system of FIG. 2a mainly includes eight transmitting terminals 21 respectively adapted to provide lights of different wavelengths, a plurality of optical amplifiers 23 each adapted to amplify transmission light, and a receiving terminal 22 for receiving the transmission light. NZ-DSFs 24 are arranged between the transmitting terminals 21 and the receiving terminal 22.
Each of the NZ-DSFs 24 used in the optical transmission system of FIG. 2a exhibits an average dispersion of 3.0 ps/nm-km. The average dispersion is a value obtained by dividing a dispersion value accumulated during the transmission of an optical signal by a transmission distance. Each NZ-DSF 24 exhibits an accumulated dispersion value of about 240 ps/nm at a point of 80 km. This accumulated dispersion value is compensated for by a DSF 25 having a dispersion value of xe2x88x92240 ps/nm.
FIG. 2b is a graph depicting a variation in accumulated dispersion value exhibited when an optical signal is traveled for a distance of 80 km in the optical transmission system of FIG. 2a. Referring to FIG. 2b, it can be found that the accumulated dispersion value increases continuously in a linear fashion as the length of the NZ-DSF increases.
FIG. 3a is an eye diagram of an optical signal transmitted in the optical transmission system illustrated in FIG. 2a. As apparent from FIG. 3a, the eye of the optical signal is unclear, and partially opened. That is, the optical signal is in a severely degraded state. Such a signal degradation is mainly caused by a four-wave mixing phenomenon. FIG. 3b illustrates the optical spectrum of an optical signal transmitted in the optical transmission system of FIG. 2a. Referring to FIG. 3b, it can be found that a signal spectrum not associated with the transmitted optical signal is generated at portions of the optical signal indicated by the reference numeral 35. Such a signal spectrum is generated due to a four-wave mixing phenomenon. Where a WDM optical transmission system using NZ-DSFs uses a narrow channel spacing of 50 GHz, its transmission quality is severely degraded due to a four-wave mixing phenomenon. This can be found by referring to FIG. 3b. 
Meanwhile, FIG. 4b is an eye diagram of an optical signal transmitted in an optical transmission system having the same configuration as that of FIG. 2a, except that optical fibers each having an increased dispersion value of 6 ps/nm-km are used in place of the NZ-DSFs in order to suppress the generation of a non-linearity. FIG. 4b illustrates the is optical spectrum of the optical signal. In this case, it is possible to suppress more or less the occurrence of a four-wave mixing phenomenon because the dispersion value of the optical fibers is higher than that of general NZ-DSFs (1.5 to 4 ps/nm-km). As a result, an improvement in signal degradation is obtained, as compared to the case of FIG. 3a. However, the eye of the optical signal is still incompletely opened due to a four-wave mixing phenomenon. In this regard, it can be found, from FIG. 4, that optical fibers exhibit an average dispersion value of 6 ps/nm-km or more so that they can be used in an optical transmission system having a reduced channel spacing of about 50 GHz.
In the case using conventional NZ-DSFs, therefore, it is impossible to perfectly transmit optical signals due to a four-wave mixing phenomenon occurring when the reduced channel spacing of 50 GHz is used for an increase in transmission capacity. This result causes a limitation on the maximum transmission capacity of the optical transmission system.
In this regard, where a WDM optical transmission system is desired to reduce the channel spacing to 50 GHz for an increase in the transmission capacity.per optical fiber, it is important to develop a fiber optic cable having further improvements in dispersion characteristics and suppression characteristics for the four-wave mixing phenomenon, as compared to conventional NZ-DSFs.
Therefore, an object of the invention is to solve the above mentioned problems involved in the related art, and to provide a fiber optic cable capable of being applied to a WDM optical transmission system using a reduced channel spacing while maximizing the transmission capacity per optical fiber in the WDM optical transmission system.
Another object of the invention is to provide a WDM optical transmission system using the fiber optic cable adapted to accomplish the above mentioned object of the invention, which uses a channel spacing of 50 GHz while having division multiplexing (WDM) optical transmission system including a plurality of connected fibers is formed of a plurality of optical fibers the fiber optic cable.
In optical fibers, wherein each of the connected increased transmission capacity by virtue of the use of optical order to an invention provides a fiber optic cable for a wavelength accomplish this object, in accordance with one aspect, the present respectively exhibiting different dispersion values and different dispersion slopes in a predetermined operating wavelength range while having different lengths and different effective areas, the optical fibers being connected to one another in an optional order.
In accordance with another aspect, the present invention provides a fiber optic cable for a WDM optical transmission system including a plurality of connected optical fibers, wherein each of the connected optical fibers comprises: first optical fiber exhibiting a first dispersion value and a first dispersion slope in a predetermined operating wavelength range while having a first length and a first effective area; and a second optical fiber exhibiting a second dispersion value and a second dispersion slope in the predetermined operating wavelength range while having a second length and a second effective area; the first and second optical fibers being connected together in an optional order.
In accordance with another aspect, the present invention provides a fiber optic cable for a WDM optical transmission system including a plurality of connected optical fibers, wherein each of the connected optical fibers comprises: a first optical fiber exhibiting a first dispersion value and a first dispersion slope in a predetermined operating wavelength range while having a first length and a first effective area; a second optical fiber exhibiting a second dispersion value and a second dispersion slope at the predetermined operating wavelength range while having a second length and a second effective area; and a third optical fiber exhibiting the first dispersion value and the first dispersion slope at the predetermined operating wavelength range while having a third length and the first effective area; the first optical fiber, the second optical fiber, and the third optical fiber being connected to one another in this order.
The present invention also provides a WDM optical transmission system having a predetermined channel spacing and a predetermined number of channels, comprising: a transmitting terminal for providing a plurality of optical signals respectively having different wavelengths; a multiplexer connected to the transmitting terminal and adapted to multiplex the optical signals; a plurality of fiber optic cables each including a plurality of connected optical fibers, each of the connected optical fibers being formed of a plurality of optical fibers respectively exhibiting different dispersion values and different dispersion slopes in a predetermined operating wavelength range while having different lengths and different effective areas, the optical fibers being connected to one another in an optional order; connecting means for interconnecting the fiber optic cables; optical amplifiers for amplifying the optical signal being transmitted through the fiber optic cables; a demultiplexer for demultiplexing the optical signal transmitted through the fiber optic cables; and a receiving terminal connected to the demultiplexer and adapted to receive the demultiplexed optical signal.
In accordance with another aspect, the WDM optical transmission system has a channel spacing of 50 GHz.
In accordance with the present invention, the connected optical fibers, each of which is formed of optical fibers of different kinds connected to one another, exhibit dispersion characteristics capable of sufficiently suppressing the occurrence of a four-wave mixing phenomenon. Thus, the present invention can provide a WDM optical transmission system having an increased transmission capacity while operating without involving any signal distortion.
In accordance with the present invention, the connected optical fibers of the fiber optic cable are formed by connecting optical fibers, respectively having dispersion values of opposite signs, in an alternating fashion. In accordance with such a connection between optical fibers of different kinds, a high positive accumulated dispersion value is first exhibited during the transmission of an optical signal through the fiber optic cable. By virtue of such a high positive accumulated dispersion value, the occurrence of a four-wave mixing phenomenon is considerably suppressed. The high positive accumulated dispersion value is then rapidly compensated for as the optical fiber having the positive dispersion value is connected to another optical fiber having a negative dispersion value. Such high accumulated dispersion value generating and compensating procedures are repeatedly carried out. By virtue of the compensation for the high accumulated dispersion value, accordingly, a desired average dispersion value is maintained in every fiber optic cable. By virtue of the high accumulated dispersion value repeatedly exhibited during a travel of the optical signal, the occurrence of a four-wave mixing phenomenon is continuously suppressed.
However, where each of the optical fibers having the same dispersion value has an increased length of 10 km or more, there is a difficulty in the manufacture and installation of the fiber optic cable because the length of the fiber optic cable is 20 km or more. In this connection, the present invention provides a fiber optic cable having an optimum length and an optimum dispersion value capable of suppressing the occurrence of a four-wave mixing phenomenon while allowing a practical manufacture of the fiber optic cable.
Therefore, the fiber optic cable according to the present invention can sufficiently suppress the occurrence of a four-wave mixing phenomenon while exhibiting an average dispersion value equivalent to that of the conventional NZ-DSF.
In addition, the fiber optic cable of the present invention can control the effective area of each optical fiber forming the connected optical fiber. That is, each optical fiber can have an effective area capable of suppressing the occurrence of a four-wave mixing phenomenon. Accordingly, a more effective suppression for the occurrence of a four-wave mixing phenomenon is achieved.
Since the fiber optic cable of the present invention can remarkably suppress the occurrence of a four-wave mixing phenomenon, it is possible to greatly suppress a degradation in transmission signals in a WCM optical transmission system using a reduced channel spacing for an increase in transmission capacity.